1. Field of the Invention
This invention relates to a process for purifying perfluoro(propylvinylether) and more particularly to a process for purifying perfluoro(propylvinylether) by effectively separating perfluoro(propylvinylether) from the hydrogen fluoride adduct of perfluoro(propylvinylether).
2. Description of the Prior Art
Perfluoro(propylvinylether) (FPVE) is synthesized from perfluoro(2-propoxypropionyl) fluoride by gas phase thermal decomposition or by liquid phase thermal decomposition according to the following reaction equation (U.S. Pat. Nos. 3,132,123, 3,250,808 and 3,321,532): EQU C.sub.3 F.sub.7 OCF(CF.sub.3)COF.fwdarw.C.sub.3 F.sub.7 OCF.dbd.CF.sub.2
The gas phase thermal decomposition is carried out by contacting the raw material perfluoro(2-propoxypropionyl) fluoride with an alkali metal compound such as sodium carbonate in a reaction zone kept at a high temperature such as 300.degree. to 600.degree. C. to thermally decompose the raw material. On the other hand, the liquid phase thermal decomposition is carried out by adding the raw material and an alkali metal compound such as sodium carbonate to a polar organic solvent such as ethyleneglycol, stirring the mixture at room temperature, thereby converting the raw material to an alkali metal salt thereof, and then heating the mixture to a temperature of 110.degree. to 130.degree. C. to thermally decompose the alkali metal.
In any of these thermal decomposition procedures, about 5 to about 10% by weight of heptafluoropropyl-1,2,2,2-tetrafluoroethylether C.sub.3 F.sub.7 OCFHCF.sub.3 is by-produced as a hydrogen fluoride adduct (FPVE.HF) of the product (FPVE) to inevitably contaminate the product (FPVE). Accordingly, European Patent No. 0260773 proposes to control formation of the FPVE.HF by-product by simultaneously using diethyleneglycol dimethylether and dimethylformamide as solvents in the liquid phase thermal decomposition procedure, but its effect has been found not satisfactory.
Most of the by-products can be readily separated and removed by ordinary washing and distillation, but FPVE.HF has a boiling point of 42.degree. C., which is quite near that of FPVE, i.e. 35.5.degree. C. and thus is hard to separate by the ordinary distillation. It may be also possible to convert FPVE.HF to any appropriate derivative and then separate the derivative, but the procedure including the chemical conversion is also hard to carry out and thus is not commercially practicable.